Here's why:

* Electronegativity: Fluorine has the highest electronegativity of all elements. Electronegativity is the tendency of an atom to attract electrons towards itself in a chemical bond. The higher the electronegativity, the more non-metallic the element is.

* Ionization Energy: Fluorine also has a very high ionization energy, which is the energy required to remove an electron from a neutral atom. This high energy indicates a strong hold on its electrons, further supporting its non-metallic nature.

* Electron Affinity: Fluorine has a high electron affinity, meaning it readily gains electrons. This is another characteristic of non-metals.

In summary: Fluorine's exceptional electronegativity, high ionization energy, and high electron affinity make it the most non-metallic element in the periodic table.